LAUSR

Introduction: Spinal muscular atrophy (SMA) is an autosomal recessive neuromuscular disease that results from mutation of the survival motor neuron 1 gene (SMN1) and the most common genetic cause of infant death. Approximately 95% of SMA cases are caused by a deletion in both alleles of exon 7 in the SMN1 gene. The copy number of the highly homologous SMN2 gene is an important predictor of the severity of SMA as it has been shown to decrease disease severity in a dose-dependent manner. SMN1 and SMN2 only differ by a few nucleotides, presenting a challenge in determining copy numbers. While carriers typically have one copy of SMN1, cis duplication of SMN1 can produce “silent carrier” (2 + 0) genotypes, which are often associated with two SMN1 variants, c.*3 + 80T>G and c.*211_*212del, that can improve the overall carrier detection rate. SMA treatments SPINRAZA®,, Evrysdi™, and ZOLGENSMA® achieve profound benefits on survival and motor milestones by modifying SMN2 splicing or using gene replacement with functional SMN genes. Early detection of SMA (including SMN2 copy number status) and identification of at-risk couples through carrier screening is critical to aid in early intervention and family planning decisions. We developed an accurate and robust single-tube PCR assay and companion software (AmplideX® PCR/CE SMN1/2 Plus Kit*) that uses capillary electrophoresis (CE) to quantify SMN1 and SMN2 copy numbers (0 to ≥4) and determines the presence/absence of the two SMN1 gene duplication “silent carrier” variants, c.*3 + 80T>G and c. *211_*212del, and the SMN2 disease modifier variant c.859G>C. The SMN1/2 Plus Kit has been previously validated for use with DNA isolated from blood. Here, we verify that DNA isolated from buccal swabs can also be used to determine SMN1 and SMN2 copy number and expanded content using this kit. Materials and Methods: A total of 60 DNA samples isolated from buccal swabs, with varying SMN1/2 copies and other positive and negative variants,were tested using the SMN1/2 Plus kit at a single site (Asuragen). Samples were tested in two cohorts: an initial cohort containing 17 samples isolated from buccal swabs with column or magnetic bead-based methods, and a second cohort of 43 samples isolated from matched blood and buccal samples using column-based methods. PCR products were generated using a Veriti thermal cycler and resolved on Applied Biosystems™ 3500xL, 3130xl, 3730xl, and SeqStudio™ Genetic Analyzers. Raw electrophoresis data (.fsa) files were directly imported into an assay-specific analysis module of the AmplideX® Reporter software that automates peak detection and sizebased classification, SMN1 and SMN2 exon 7 copy number quantification, detection of gene duplication and disease modifier variants, and sample- and batch-level quality control checks. Samples were analyzed using the default (kit calibrator) and user-defined calibration (UDC) (buccal DNA) workflows as described in the protocol. Results: For the initial cohort of 17 Buccal swab samples, SMN1 copy number calls were concordant with MLPA reference results (reported as 0, 1, 2, or ≥3) for 16/17 (94.1%) of samples with default calibration and 17/17 (100%) of samples with UDC. Further, concordance for carrier samples (1 SMN1 copy) were 7/7 (100%) using both methods. SMN2 copy numbercallswere concordant with MLPA reference results for 17/17 (100%) of samples with either default calibration or UDC. For the second cohort of 43 buccal swab samples with matched blood samples, SMN1 and SMN2 copy number calls were concordant with the results from the paired whole blood for at least 95% of samples assessed across the four different CE platforms. All variant status calls were concordant between the buccal swab and whole blood results. Conclusions: Here, we demonstrate that buccal swabs are a compatible DNA source for the quantification of 0, 1, 2, 3, and ≥4 gene copies of both SMN1 and SMN2 and the status determination of three clinically significant variants using the single-tube PCR/CE SMN1/2 Plus kit. Although d fault calibration yielded high rates of agreement between copy number results from buccal swabs and reference results, analyzing samples with user-defined calibration (i.e. calibrating to a buccal swab sample) modestly improved concordance. These results suggest that DNA samples isolated from buccal swabs are compatible with this assay and has implications for more facile sample collection and handling, particularly given the strain of COVID-19 on healthcare infrastructure.